The present invention relates to an electrical storage device electrode binder composition, an electrical storage device electrode slurry that includes the binder composition and an electrode active material, an electrical storage device electrode that is produced by applying the slurry to a collector, and an electrical storage device that includes the electrode.
In recent years, a high-voltage electrical storage device having a high energy density has been desired as a power supply for driving an electronic instrument. In particular, a lithium-ion battery or a lithium-ion capacitor is expected to be a high-voltage electrical storage device having a high energy density.
An electrode used for such an electrical storage device is produced by applying a mixture of an electrode active material and polymer particles that function as an electrode binder to the surface of a collector, and drying the mixture. The polymer particles are required to exhibit a capability of binding the electrode active material, a capability of binding the electrode active material and the collector, scratch resistance when winding the electrode, and powder fall resistance (i.e., a fine powder of the electrode active material or the like does not occur from an electrode composition layer (hereinafter may be referred to as “electrode active material layer”) due to cutting or the like). If the polymer particles satisfy these properties, it is possible to produce an electrical storage device that has high flexibility in structural design (e.g., an electrode folding method or an electrode winding radius), and can be reduced in size. It was found that a capability of binding the electrode active material, a capability of binding the electrode active material and the collector, and the powder fall resistance have an almost proportional relationship. Therefore, these properties may be comprehensively referred to as “adhesion”.
When producing a positive electrode, it is advantageous to use a fluorine-containing organic polymer (e.g., polyvinylidene fluoride (PVDF)) that exhibits slightly poor adhesion, but exhibits excellent oxidation resistance. When producing a negative electrode, it is advantageous to use a (meth)acrylic acid polymer that exhibits slightly poor oxidation resistance, but exhibits excellent adhesion.
Various techniques have been studied and proposed in order to improve the properties (e.g., oxidation resistance and adhesion) of a polymer that is used for the electrode binder. For example, JP-A-2011-3529 proposes a technique that provides a negative electrode binder with oxidation resistance and adhesion by utilizing PVDF and a rubber polymer in combination. JP-A-2010-55847 proposes a technique that improves adhesion by dissolving PVDF in a specific organic solvent, applying the solution to the surface of a collector, and removing the organic solvent at a low temperature. JP-A-2002-42819 proposes a technique that improves adhesion by utilizing an electrode binder having a structure that includes a main chain formed of a vinylidene fluoride copolymer and a side chain that includes a fluorine atom.
A technique that improves the above properties by controlling the composition of a binder (see JP-A-2000-299109), and a technique that improves the above properties by utilizing an epoxy or hydroxyl group-containing binder (see JP-A-2010-205722 and JP-A-2010-3703), have also been proposed.
The negative electrode binder disclosed in JP-A-2011-3529 that utilizes a fluorine-containing organic polymer and a rubber polymer exhibits improved adhesion, but the oxidation resistance of the organic polymer deteriorates to a large extent. Therefore, an electrical storage device produced using the negative electrode binder has a problem in that the charge-discharge characteristics irreversibly deteriorate due to repeated charge-discharge cycles. The electrode binder disclosed in JP-A-2010-55847 or JP-A-2002-42819 that utilizes only a fluorine-containing organic polymer cannot achieve sufficient adhesion.
The binder composition disclosed in JP-A-2000-299109 or JP-A-2010-205722 exhibits improved adhesion. However, it is difficult to maintain good charge-discharge characteristics for a long time since the binder that adheres to the electrode active material functions as a resistance component of the electrode.
These electrode binder compositions are merely evaluated in terms of the properties of an electrical storage device, and the storage stability of the electrode binder compositions that is important for practical use has not been studied.